1. Field of the Invention
This invention relates to a process for production of hydrogels and more particularly to a process for producing hydrogels capable of absorbing a large amount of water which are suitable for use as water-absorbing material or as a gel material for use in a swollen state.
2. Description of the Prior Art
Nonwoven fabrics, paper, pulp, spongy urethane resins, sponges, etc. have hitherto been used as water-absorbing materials, e.g., in physiological articles, diapers, throwaway dusters, etc. These materials, however, are not necessarily satisfactory, often because of insufficient water-absorbing capabilities.
Recently, in place of such water-absorbing or water-retaining materials, materials have been proposed which are produced by crosslinking water-soluble polymeric substances with crosslinking agents or by convertion to a water-insoluble nature by replacement of part of the hydrophilic groups by lipophilic groups. For example, such materials are known to be produced by: copolymerizing acrylamide with polyfunctional compounds such as N',N'-alkylidenebisacrylamide, ethyleneglycol diacrylate and divinylbenzene (Japanese Pat. No. 23462/1968); crosslinking N-hydroxyacrylamide based polymers using light energy at low temperatures (Japanese Pat. No. 31823/1969); copolymerizing at least 50% by weight polyol methacrylate, monomers containing acidic groups, and monomers containing basic groups in the presence of polyfunctional compounds such as ethyleneglycol diacrylate, methylenebisacrylamide or divinylbenzene (Japanese Pat. No. 25749/1973); saponification of crosslinked polyethyleneglycol, crosslinked polyvinyl pyrrolidone, crosslinked sulfonated polystyrene and starch-acrylonitrile graft-copolymers (U.S. Pat. Nos. 3,661,815 and 3,669,103); preparing mixtures of saponified starch-acrylonitrile graft copolymers and soft polymers having a low glass transition temperature (Japanese Patent Application (OPI) No. 75747/1976 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application")); reacting saponification products of ethylene-vinyl ester-ethylenically unsaturated carboxylic acid or derivative copolymers thereof with crosslinking agents (Japanese Patent Application (OPI) Nos. 104651/1978 and 104652/1978), copolymers of starch, cellulose or the like with acrylic acid salts or difunctional compounds (Japanese Patent Application (OPI) Nos. 130788/1978 and 130789/1978), etc.
However, when such materials are used practically, various problems occur. For example, some are insufficient in water-absorbing capability, some are complicated with respect to the production methods thereof, and when they are used in the state that they contain water therein (i.e., are in the wet state) for long periods of time, the gel structure is broken (i.e., they are thus of low stability).